Rapid communication is essential in modem business, and a world wide communication system by mobile communication technology, including satellite stations, is becoming more readily accessible for civilian uses.
Commercial opportunities in the consumer field cannot be developed unless the cost of producing microwave communication equipment is lowered and their reliability increased. An important step in achieving such an objective is monolithic microwave integrated circuit (MMIC), which could offer a potential for high circuit packing density, reliability and low cost. There are many barriers to overcome before such an objective can be achieved, however, and a major one is the deviation of circuit performance from the design specifications, because of the many problems involved in fabricating the devices which are based on GaAs substrates.
At high frequencies associated with microwave operations, a number of technical problems arises to prevent a direct application of the manufacturing technology based on the conventional silicon-IC fabrication. The performance requirements dictate that the substrate material be of high mobility, insulating gallium arsenide. Signal loss in microwave circuits can be caused by two effects: localization of signals in the surface of a conductor close to the ground line; and another localization effect known as the skin effect at microwave frequencies. These effects dictate that device quality and fabricability considerations be given in the early stages of circuit design. Circuit resistance and parasitic interactions present much more serious problems of signal transmission losses in GaAs-based devices than in Si-based devices.
It is also evident that practical MMIC technology requires a fundamental rethinking in the transmission line and active element designs, because of the limited choices presently available in the circuit fabrication processes developed in association with the current silicon based technology. The current state of the art of fabricating circuits in the MMIC technology is illustrated in the following.